1. Fast & Easy ECGs, 2nd E – A Self-Paced Learning Program15
AV Heart Blocks
Fast & Easy ECGs, 2nd E – A Self-Paced Learning Program

2. AV Heart Blocks 1st-degree AV heart block
2nd-degree AV heart block, type I (Wenckebach)
2nd-degree AV heart block, type II
3rd-degree AV heart block
AV dissociation
Instructional point:
When the conduction is completely blocked at the AV node the heart will generate a QRS complex from one of the lower pacemakers in the His-Purkinje conduction system or the ventricles. I

3. Heart Blocks
Partial delays or complete interruptions in the cardiac conduction pathway between the atria and ventricles
The degree of block defines the type and classification of heart block
Question to ask “What is the role of the AV node in conducting the electrical impulse through the heart?”
Answer: The AV node slows conduction from the atria to the ventricles to allow the ventricles to finish filling.
Instructional point:
When the conduction is blocked the heart will generate a QRS complex from one of the lower pacemakers. Usually this comes from the AV node but in the event the AV node is inoperable the escape pacemaker will be the Purkinje fibers in the ventricles. Q I

4. Heart Blocks Common causes: Ischemia Myocardial necrosis
Degenerative disease of the conduction system
Congenital anomalies
Drugs (especially digitalis preparations)
Instructional point:
The more significant heart blocks are those that reduce the ventricular heart and subsequently adversely affect cardiac output. I

5. 1st-Degree AV Heart Block
Not a true block
Is a consistent delay of conduction at the level of the AV node
Question to ask the students: “What is the normal duration of the PR interval?”
Answer: The normal PR interval is 0.12 to 0.20 seconds in duration.
Instruction point:
First degree AV heart block may be temporary, particularly if it stems from ischemia early in the course of myocardial infarction or certain medications. Q I

6. Causes of 1st-Degree AV Heart Block
May occur in healthy persons for no apparent reason (particularly athletes)
Sometimes associated myocardial ischemia, acute MI, increased vagal tone, and digitalis toxicity
Other causes include:

7. Effects of 1st-Degree AV Heart Block
Often of little or no clinical significance because all impulses are conducted to the ventricles
Can progress to higher degree block, especially in the presence of inferior wall myocardial infarction
Instructional point:
The patient experiencing this condition is usually asymptomatic. I

8. Treatment of 1st Degree AV Block
As cardiac output is not affected, no specific treatment is indicated, however, efforts may directed toward identifying and treating the cause

9. 2nd-Degree AV Heart Block, Type I
Intermittent block at the level of the AV node
Also referred to as Wenckebach

10. 2nd-Degree AV Heart Block, Type I
More P waves than QRS complexes and the rhythm has patterned irregularity
PR interval increases until a QRS complex is dropped
After dropped beat the next PR interval is shorter
As each subsequent impulse generated there is a progressively longer PR interval until again, a QRS is dropped
Cycle repeats
Instructional points:
The atrial rate is that of the underlying rhythm while the ventricular rate is slightly less than atrial rate (slower than normal). The QRS complexes are within normal limits. The P-P interval is constant while the R-R interval decreases until a QRS is dropped.
The type of irregularity seen with Wenckebach is also called a patterned irregularity. I

11. 2nd-Degree AV Heart Block, Type I
Often occurs in acute MI or acute myocarditis
Other causes include:

12. Effects of 2nd-Degree AV Heart Block, Type I
May occur in otherwise healthy persons
Usually transient and reversible, mostly resolving when underlying condition is corrected
May progress to more serious blocks (particularly if it occurs early in myocardial infarction)

13. Effects of 2nd-Degree AV Heart Block, Type I
If dropped ventricular beats occur frequently, patient may show signs and symptoms of decreased cardiac output
Instructional point:
Remind the students that heart rate X stroke volume = cardiac output. I

14. Treatment of 2nd-Degree AV Heart Block, Type I
Asymptomatic patients require no specific treatment
Symptomatic patients (e.g., chest pain, hypotension) should receive oxygen, an IV lifeline, and the administration of atropine and transcutaneous pacing should be considered if the heart rate is slow

15. 2nd-Degree AV Heart Block, Type II
Intermittent block at the level of the bundle of His or bundle branches resulting in atrial impulses that are not conducted to the ventricles
Instructional point:
Second degree AV block is less common than type I but it is more serious as it often progresses to complete AV heart block. It differs from Mobitz type I in that the PR interval is constant prior to a beat being “dropped.” I

16. 2nd-Degree AV Heart Block, Type II
More P waves than QRS complexes
Duration of PR interval of the conducted beats remains constant

17. Causes of 2nd-Degree AV Heart Block, Type II
Usually associated with anterior-wall MI, degenerative changes in the conduction system, or severe coronary artery disease
Common causes include:

18. Effects of 2nd-Degree AV Heart Block, Type II
A serious dysrhythmia (usually considered malignant in the emergency setting)
Can result in decreased cardiac output and may produce signs and symptoms of hypoperfusion
May progress to a more severe heart block and ventricular asystole
Instructional point:
Signs and symptoms of hypoperfusion include low blood pressure, shortness of breath, congestive heart failure, pulmonary congestion, and decreased level of consciousness. I

19. Treatment of 2nd-Degree AV Heart Block, Type II
Symptomatic patients should receive oxygen, an IV lifeline, and transcutaneous pacing
Atropine may be used in type II AV block with new wide QRS complexes while you are setting up the transcutaneous pacemaker

20. 3rd-Degree AV Heart Block
Complete block of conduction at or below the AV node
Impulses from atria cannot reach ventricles

21. 3rd-Degree AV Heart Block
Atrial pacemaker site is the SA node
Atrial rate 60 to 100 BPM
Ventricular pacemaker site is an escape rhythm
From AV junction rate 40 to 60 BPM
From ventricles rate 20 to 40 BPM
Upright and round P waves seem to “march right through the QRS complexes”
Question to ask the students: “What will the QRS complexes look like if the escape rhythm is arising from the ventricles?”
Answer: Escape pacemakers arising from the ventricles will produce wide and bizarre-looking QRS complexes.
Instructional point:
The reason why the P waves seem to march through the QRS complexes is there is no relationship between the P waves and QRS complexes. Q

22. Causes of 3rd-Degree AV Heart Block
3rd-degree AV heart block occurring at the AV node is most commonly caused by a congenital condition
It may also occur in older adults because of chronic degenerative changes in the conduction system
Other causes are:

23. Effects of 3rd-Degree AV Heart Block
Well tolerated as long as the escape rhythm is fast enough to generate a sufficient cardiac output to maintain adequate perfusion
Can result in decreased cardiac output because of the asynchronous action of the atria and ventricles and if the ventricular rate is slow
Instructional point:
Third degree AV block with wide QRS complexes is considered an ominous sign.
This dysrhythmia is potentially lethal as patients with this dysrhythmia are often hemodynamically unstable. I

24. Treatment of 3rd-Degree AV Heart Block
Symptomatic patients should receive oxygen, an IV lifeline, and transcutaneous pacing
Atropine may be used in 3rd degree AV block with new wide QRS complexes while you are setting up the transcutaneous pacemaker
Instructional point:
The reason why the P waves seem to march through the QRS complexes is there is no relationship between the P waves and QRS complexes. I

25. Differences Between AV Heart Blocks

26. Remember!
2nd- and 3rd-degree AV heart block can lead to decreased cardiac output if the ventricular rate slows sufficiently

27. AV Dissociation
AV dissociation occurs when the atria and the ventricles are under the control of separate pacemakers and beat independently of each other

28. AV Dissociation
The key difference between AV dissociation and 3rd degree AV heart block is the ventricular rate in AV dissociation is nearly the same or faster than the atrial rate whereas in 3rd degree AV heart block it should be slower

29. Causes of AV Dissociation
Slowed or impaired sinus impulse formation or SA nodal conduction
Impulse formation in the AV junction or ventricles that is faster than the firing rate of the sinus node
Complete AV heart block
Anything that interferes with the ability of an atrial impulse to conduct to the ventricles, such as pauses produced by premature beats

30. Effects of AV Dissociation
Signs and symptoms vary depending on the underlying cause
Signs of decreased cardiac output may be present if the heart rate is reduced

31. Treatment of AV Dissociation
No treatment is needed if the condition causing AV dissociation is clinically insignificant
If the condition causing the AV dissociation reduces cardiac output, treatment is directed at managing the underlying problem
Specific measures include delivering atropine and other rate-accelerating agents and/or pacemaker insertion
If drug toxicity caused the original disturbance, the drug should be discontinued

32. Practice Makes Perfect
Determine the type of dysrhythmia
Answer: Atrial rate 56 BPM, Ventricular rate 56 BPM, regular rhythm, P waves are upright and normal and each is followed by a QRS complex, QRS complexes are normal at 0.08 seconds in duration, PRI has a longer than normal duration 0.30 seconds, QT 0.40 seconds. Sinus bradycardia with 1st-degree AV heart block. I

33. Practice Makes Perfect
Determine the type of dysrhythmia
Answer: Atrial rate 60 BPM, Ventricular rate 50 BPM, patterned irregularity, normal and upright P waves but not all are followed by a QRS complex, normal QRS complexes at 0.08 seconds, PRI is progressively longer until a QRS complex is dropped and then the cycle starts over again, QT seconds. 2nd-degree AV heart block, type I. I

34. Practice Makes Perfect
Determine the type of dysrhythmia
Answer: 50 BPM (ventricular) 84 BPM (atrial), regular rhythm, P waves are upright and normal but there are more P waves than QRS complexes, QRS complexes are bizarre-looking and wide at 0.20 seconds in duration, PRI is immeasurable (as the P waves are not associated
with the QRS complexes), the P waves appear to march through the QRS complexes, QT 0.48 seconds. 3rd-degree AV heart block. I

35. Practice Makes Perfect
Determine the type of dysrhythmia
Answer: Atrial rate 58 BPM, Ventricular rate 60 BPM, regular rhythm, P waves are upright and normal and each is followed by a QRS complex, QRS complexes are normal at 0.12 seconds in duration, PRI has a longer than normal duration seconds and constant, QT seconds. Sinus bradycardia with 1st-degree AV heart block. I

36. Practice Makes Perfect
Determine the type of dysrhythmia
Answer: Atrial rate 94 BPM, Ventricular rate 48 BPM, regular rhythm, P waves are upright and normal but there are two P waves preceding each QRS (in other words, not all the P waves are followed by a QRS complex), QRS complexes are normal at 0.08 seconds in duration, PRI seconds and is constant for the conducted beats, QT 0.36 seconds. 2nd-degree AV heart block, type II. I

37. Practice Makes Perfect
Determine the type of dysrhythmia
Answer: Atrial rate 75 BPM, Ventricular rate 36 BPM, regular rhythm, P waves are upright and normal but there are more P waves than QRS complexes, QRS complexes are bizarre-looking and wide at 0.16 seconds in duration, PRI is immeasurable (as the P waves are not associated with the QRS complexes), the P waves appear to march through the QRS complexes, QT 0.36 seconds. 3rd- degree AV heart block. I

38. Practice Makes Perfect
Determine the type of dysrhythmia
Answer: Atrial rate 45 BPM, Ventricular rate 45 BPM, regular rhythm, normal and upright P waves and each is followed by a QRS complex, QRS complexes at 0.08 seconds, PRI 0.44 seconds and constant, QT 0.48 seconds. Sinus bradycardia with 1st-degree AV heart block. I

39. Practice Makes Perfect
Determine the type of dysrhythmia
Answer: Atrial rate 94 BPM, Ventricular rate 48 BPM, regular rhythm, P waves are upright and normal but there are two P waves preceding each QRS (in other words, not all the P waves are followed by a QRS complex), QRS (more accurately described as RS) complexes are
0.08 seconds in duration, PRI seconds and is constant for the conducted beats, QT seconds. 2nd-degree AV heart block, type II (there is also a 1st degree AV block). I

40. Practice Makes Perfect
Determine the type of dysrhythmia
Answer: Atrial rate 115 BPM, Ventricular rate 42 BPM, regular rhythm, P waves are upright and normal but there are more P waves than QRS complexes, QRS complexes are normal at 0.12 seconds in duration, PRI is immeasurable (as the P waves are not associated with the QRS complexes), the P waves appear to march through the QRS complexes, QT seconds. 3rd-degree AV heart block. I

41. Summary
Heart blocks are partial delays or complete interruptions in the cardiac conduction pathway between the atria and ventricles
1st-degree AV heart block is a consistent delay of conduction at the level of the AV node which results in a PR interval that is greater than 0.20 seconds in duration
2nd-degree AV heart block, Type I is an intermittent block at the level of the AV node

42. Summary
With 2nd-degree AV heart block, type I, the PR interval increases until a QRS complex is dropped. After the dropped beat the next PR interval is shorter. Then as each subsequent impulse is generated and transmitted through the AV junction there is a progressively longer PR interval until again, a QRS is dropped. This cycle can repeat itself
With 2nd-degree AV heart block, type I, there are more P waves than QRS complexes and the rhythm is regularly irregular

43. Summary
2nd-degree AV heart block, type II is an intermittent block at the level of the bundle of His or bundle branches resulting in atrial impulses that are not conducted to the ventricles
With 2nd-degree AV heart block, type II, there are more P waves than QRS complexes and the duration of PR interval of the conducted beats remains the same (are constant)
3rd-degree AV heart block is a complete block of the conduction at or below the AV node and impulses from the atria cannot reach the ventricles

44. Summary
In 3rd-degree AV heart block the pacemaker for the atria arises from the SA node while the pacemaker for the ventricles arises as an escape rhythm from the AV junction or from the ventricles
With 3rd-degree AV heart block the upright and round P waves seem to “march right through the QRS complexes" revealing that there is no relationship between the P waves and QRS complexes
2nd- and 3rd-degree AV heart block can lead to decreased cardiac output

45. Summary
AV dissociation occurs when the atria and the ventricles are under the control of separate pacemakers and beat independently of each other
The key difference between AV dissociation and 3rd degree AV heart block is the ventricular rate in AV dissociation is nearly the same or faster than the atrial rate whereas in 3rd degree AV heart block it should be slower